Whistle after toilet, wiping bottom and behind (6)
I believe the answer is:
losing
'behind' is the definition.
(synonyms)
'whistle after toilet wiping bottom' is the wordplay.
'whistle' becomes 'sing' (synonyms).
'after' is a charade indicator (letters next to each other).
'toilet' becomes 'loo' (loo can informally mean a toilet).
'wiping bottom' means to remove the last letter (the final or bottom letter is erased).
'loo' with its final letter taken off is 'lo'.
'sing' after 'lo' is 'LOSING'.
'and' acts as a link.
(Other definitions for losing that I've seen before include "Opposite of 9" , "Confusing" , "Mislaying or being defeated" , "Logs in (anag.)" , "Misplacing or being defeated" .)